Method of electropolishing



Sept. 9, 1952 s. E. EATON 2,610,143

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METHOD OF' ELECTROPOLISHING Filed Feb. 8, 194'? 6 Sheets-Sme?I 6 Z//vc' 001.0 /PaLLfo Y /fc/V VOL 715 @gf a juve/Zvw ggf@ Mwbm Patented Sept. 9, 1952 METHOD OF ELECTROPOLISHING Samuel E. Eaton, Lexington, Mass., assigner, by mesne assignments, to Oneida, Ltd., Oneida, N. Y., a corporation of New York Application February 8, 1947, Serial No. 727,452

. 1 This invention relates to a method ofelectropolishing metal surfaces.

In my copending application Serial No. 47.4,- 437, filed February 2, 1943, now U. S. Patent No. 2,416,294 (the disclosure of which is hereby incorporated-.byreference `and of which this application isaccordingly a` continuation-impart) a method of polishingr metalsis described wherein the metal surface to be polished is made the anode in an aqueous electrolyte containing a soluble cyanide and subjected to anelectrical potential difference (i. e., the terminal voltage measured between the electrodes of the `electrolytic cell) which is between` that at which the metal surface manifests a polishing effect and that fat which a spontaneous flashing phenomenontakes place or permanent film formation occurs. The potential is then maintained substantially constant until the Imetal surface `acquires a polished surface, for example in a quiescent bath.. This result may be further promoted by agitation iof the anode orl Iof the electrolyte, without turbulence, and by preventing too close anv approach of the anode to other solids.

It is now discovered thatwhile a controlled and constant potential difference across the electrolytic cell is a useful and desirable method for electropolishing of the metal anode, when commercial application of the method ismade the `practical problem of accurately maintaining the potential at a desired point is sometimes presented.

By the present invention it is found that a potential difference across the cell which is posi-` tively uctuated between predetermined limits of voltage and current density and in predetermined periods offluctuation, results in an improved commercial method yielding `reproducible polished surfaces comparable to those obtained by the method of holding the potential constant. The factor that is thus left to be closely controlled is that 'of time which, for commercial practice, isless costly than accurate voltage control.

When a metal surfacewhich may, .for example, be a silver surface which has been formed by electroplating, in which operation it constituted the` cathode-is made the anodeof an electrolytic cell, containing an alkaline cyanide in an aqueous electrolyte, and the imposed potential is varied from zero upward, various changes in the appearance of the surface may be observed.

`Considering the surface of only a very small.

area ofthe above anodel when of silver, for eX- ample, the following sequence of changes ap- 16 claims. p (o1. ani- 1405 pears to' take place. As the voltage is increased,

the current density increases and the surface is etched tov a light grey color which remains up to apoint called herein the polarization point. This point marks a change in therelationship between further increase in potential with respect to increasing current density in that the current density no longer continues to increase at the same rate. `It may increase, butat a slower rate, or it may even not increase at all. If maintained at potentials slightly above the polarization point the etched appearance of the surface changes to one of specular brightness and coveringthe surface is alight brown colora` tion which may be hardly visible. If the potential is increased, and the polished area is closely observed, the brown coloration will be seen to become darken'thoughif the current is cut oif or the anode removed from solution-such discoloration disappears leavingithe `surface polished. As the potential israised still further, a peculiarly unstable condition is usually encountered which is termed spontaneous flashing. In this region the brown discoloration noted above repeatedly and spontaneously fades and returns givingthe appearance of flashing. The appearance when faded is one of light brown or grey.` Above this potential there is a `gradual change to a light grey coloration which may be due to an etching effect; and upon increasing the potentiaLit becomes darker. These later colorations which are formed at potentials above the flashing point, as contrasted with the 'brown coloration discussed previously, are permanent-and especially that coloration which occurs when the anode is maintained at a suflcient potential and for a sufficient time tending to produce the dark grey coloration.v By permanent is meant that the coloration does not readily or completely disappear when the current is cut off or the anode removed from the cell.

The above discussion relates to a` given very small area of a silver anode surface. In practice itis observed that corners and sharp edges of the anode become Apolarized first because of higher current density at these` points. Upon gradually increasingthe potential the brightening and other effects on theseparts as described above appear to precede such changes on other parts which are of lower current density. Thus in the case of spoons the edges would exhibit the sequence of change first and the concave center of the bowl would pass` through the sequence and hence receive its polish last.

The observed relationships of potential to current density with respect to the electrodes and the cell as a whole are therefore averages of thc varying effects taking place throughout the entire surface of the anode. This illustrates why if voltage alone is controlled and constant, the potential at which all parts of a spoon, for instance, are brightened at one and the same time, is quite critical. It also illustrates the fact that polishing action begins to take place on at least part of the anode at a voltage below that required to polarize the entire anode.

By the present invention it is found that by imposing a positively controlled predetermined liiuctuation of the potential difference between the anode and the cathode of the cell through at least a part of the range between the region of polarization and the region of permanent discoloration but without maintaining the potentialv above or below this range for any prolonged period or predominant proportion of the be polished in the depression and the formation of soluble cyanidesV on the protuberances.

But by whatever specific mechanism the polishing operates this is the result, that the anode surface progressively develops a dependable and reproducible polish by the imposition of a positively controlled and fiuctuated potential thereon as above described, asV contrasted with an unreproducible surface if uncontrolled voltages or current densities are allowed, as in the spontaneous flashing phenomenon.

.Agitation of the electrolyte (preferably without turbulence of the electrolyte) appears to lower the resistance (increase the conductivity) of the system, causing a higher current density at a'given potential, and effectively promotes the polishing action of the' cell.

The process may be carried out by varying the voltage in any of a number of ways. There are, however, certain limitations which should be observed. Excessive time either in the region below the polarization point or in the region of permanent dis-coloration is to be avoided. While theY time period or periods of fluctuation may bevaried, no advantage is obtained by too long a period of iiuctuati'on.v Short periods of fluctuation 'are sometimes more effective to develop the polishing action and the polished surface on the anode most satisfactorily.

The principal advantage of the present process is the elimination of the necessity for close control of a constantly maintained voltage. Moreover, the positively controlled fluctuation of the voltagerinto the regions above and/or below the polishing range are not only permissible but found to be advantageous because easier from the standpoint of practical operations, It has been foundk that best polishing results are obtained if the voltage is not within such regions for a prolonged or dominant proportion of the time of` each uctuation period but lis varied substantially all the operating time and predominantly or completely within the polishing range.

Thus the process of first applying voltage at any point above the polarization point and then cutting the current off, intermittently, will give polishing. This is an extrapolated method of accomplishing applicants process, but such procedure is not to be preferred over a positive voltage maintained at all times, though it need not necessarily be maintained at all times above the polarization point.

Under some practical plant conditions it may be found desirable to fluctuate the potential in such a manner that the voltage never falls below the polarization point but is fiuctuated upwardly into the region of permanent nlm formation to a considerable extent. It is also possible to cut the voltage to zero or substantially zero for part of each cycle, but after the voltage reaches zero on the anode surface no polishing takes place and such subsequent portion of the fiuctuating period can be disregarded as forming partv of the uctuation cycle. 2

Typical examples of the invention were'carried out in Which the potential was gradually increased and the values of the voltage and cur-V Fig. 1 presents the results obtained with a silyau,y

cyanide electrolyte containing 0.3 molal silver and about 0.29 molal'free'c'yanide (above that required to form KAg (CNM with the silver present) and 0.1 molal KzCOa, by imposing a gradually increasing potential difference between -the anode and cathodes in a quiescent bath.

Fig. 2 shows results obtained with silver in an electrolyte containing 5% KCN (about 5/5 molal) with no appreciable silver cyanide or potassium carbonate, in a quiescent bath.

Fig. 3 shows results forrthesame ,5%V KCN bath but here the silver anode is agitated in a continuous circular path at a linear speed of 14 ft. per minute. A

Figs. 4 to 8 inclusive are for the following metals in 5% KCN baths: bronze, copper, cadmium, zinc, and nickel silver, respectively.

Figures 9 to 28 are graphs showing the characteristic fiuctuations of voltage with time, with respect to the several types of metal surfaces to be polished, the compositions of the electrolytic baths employed, with or without agitation, and the results obtained thereby.

Referring to Fig. 1, in particular relating to silver, region A represents the low voltage and low current density region in.which etching predominates, region B is the polishing region, and region C, the flashing and/or permanent coloration region. As previously explained, though these regions are .sharply separated on the graphs-in general, the effects in one may persist into the near parts of the adjacent regions.

Regions B and C of graph I have been subdivided typically into subdivisions B1, B2 and into C1, C2 and C3, respectively. The upper region of permanent color formation C shows three slightly different effects in the subdivisions labelled C1, C2 andCz, respectively.A In C3, while the current is on, a definitely dark grey coloration gradually forms which is permanent andv may become 5, physically separable from `the solid metal surface. If voltage is applied suddenly in this region, a period of time,isometime's a few'secondsfis` re quired before the coloration becomes permanent. The surface turns` a darlcgrey .when` first `withdrawn4 from the bath.. Parton the dark grey iilin turns to a light greyoand thesilver surface appears to be etched under the microscope but part of the dark grey film remains-dark and withstands considerable abrasion and withstands at least a few minutesfcontact with nitric acid. This dark permanent nlm can be scratched oi of the surface-*with a probe and -under `the microscope appears to bea black precipitateprobably' of silver oxide. i

In Czvfa permanent coloration is formed `after asshort'lt'ime, which is grey-to dark greyand which does not readily redissolve or disappear upon sharp'reduction of the-potential or cutting oiof the current altogether. The lighter grey sometimes appears to be a line `etching of the surface. It also represents the preferred upper limit for the positively imposed fluctuations of potential for polishing purposes to avoid the tendencies of permanent. film formation.

In C1 the anode mayupon continued maintenance of the potential undergo the spontaneous flashing phenomenon over the whole surface l'or parts of it and ,with very rapid` alternations to very slow alternations, but without accurate prediction, duplication or control. Such dashing may be arrested by, for example, shutting offthe current or byincreasing the potential up into or above the regions, C2 or C3 or down into region B2, B1 01* A.

The electrical .power source in the particular instances above plotted had a rated capacity of `amperes, 6 volts D. C. The voltage and am-f perage simultaneously fluctuated with the spontaneous fllashing. effect. Thus as the brown discoloration fades; the current; rises and the voltage falls. Then. the coloration reappears, the current falls and the voltage rises. This is what might be expected if the coloration were an in-` sulating film. This `cycle of changes is represented in- Fig. 1 by the sloping dotted lines'ofy region C1; each vdotted line schematically represents the change in thevoltage-amperage relationship during the -fiashing cycle. Each line corresponds to a different setting of the applied voltage source.

In zone VB, two polishing effects are Ynoted `and labelled B1 and Bz'. In subdivisionBz the entire anode develops a uniform polish. A browncoloration covers the surface. This colorationl is darkV brown at the higher voltages of region B2 and lighter brown at lower voltages. If the cur-` rent is lowered or the anode removed from the solution this coloration disappears to leave the surface polished.

The subdivision B1 is generally that range of potential differences in which the anode surface shows tendencies to acquire polishingpreferentially at the margins or corners while thecenter is usually atglower current density and thus. is less polished than the edges.

The other graphs show similar regions A, B an Cl which have the same general characteristics as` those described with` reference to Fig. 1. The flashing phenomenonywas not noted however and therefore does not appear in the graphs.

The boundaries between different regions are not; sharp because the effect in one region graduates somewhat into that of the next, as previousiy mentioned. l

Polishing operations were carried out on;l silver anodes vinelectrolytes orxcompositons; as givenl in the legends indicated upon theaccompanyinggraphs, with various` ranges of. imposed iluctuat. tions of potential, as indicated below.`

during faperiod. of aboutl 5. seconds is gradually' raised to a voltage of 1.5. During the next 5 secondsfit is gradually lowered to, 1.1; during the next 5 seconds it is again raised to 1.5, etc. This cycle is repeated for a total of 2 minutes (see Fig.' 9). It was noticed .that the brown coloration which'appeared while the current was on. varied in intensity ofl color when the voltage was varied and disappeared completely and. almost .instantaneously when the. current was shut oii. 'I'he silver when taken from the bath had a polish which was rated as 1A mirror on a somewhat arbitrary scale;` that. is, as one looked at its surface, reflected images couldbe seen with a clarity that was estimated to. be about half that of 'a perfect mirror.

Ina similar bath and with no agitation the voltage was varied in the same manner except that the voltage was varied from .6, volt `to 1.3 volts, in the 5 second periods (see Fig. 10). Polishing was not as goodat this lower voltage range but the edges were polished. The brown discoloration swept `on and off` the surface las the voltage was varied. i

The voltage wasalso varied in another way and quite similar results obtained.` For examplathe.

voltage was maintained at 1.1` volts for 21/2 seconds. It was thenchanged rapidly to 1.5 volts and maintained there for about 21/2 seconds.

It was then quickly lowered kto 1.1 volts. and maintained for v21/2 seconds, etc. (see Fig. 11).

This was repeated for a total time of 2 minutes' and it was noticed thatV the lighty brown discoloration which was present when the current was on in the solution` again varied in intensity as the voltage varied and the silver when with.`

variations. The short ,time at` 1.9 volts was not suiiicient .to produce a damaging permanent discoloration and when the anode was removed. it. had a lustrous surface although not mirror perfect in the two minutes.

The` same voltages were` applied` in the same.

manner except that they were maintained for 15 seconds in another run and essentially similar results obtained (see Fig.l 13).

The voltage was raised to 1.9 volts and` maintained there for 2 seconds.. The current was then. cut oi completely for 2 sec0nds, a Voltage of 1.9 again applied for 2 seconds and again` cut. olyetc. for a total time of 2` minutes (see Fig. 14).l

`In. solution the` anode was greyfwhen the current.,-

The anode had a brown color inr solution which varied 'in intensity with voltage face was lustrous.

Fig. 2 illustrates typical voltage-amperage relationships yfor a silver anode in a 5% KCN bath with no agitation. A polish Was obtained by applying a voltage of 3.5 for 5 seconds, rapidly raising to 3.7 volts and holding there for 5 seconds, then rapidly lowering to 3.5volts for `5 seconds, etc. (see Fig. 15). The anode showed a. brown discoloration when in the solution and this discoloration varied in intensity as the voltage was varied. Similar results were obtained in an identical experiment which was like the above except that the time at 3.7 volts was l seconds instead of the time at 3.5 volts was 5 seconds (see Fig. 16).

- Fig. 3 illustrates voltage amperage relationship for a silver anode in a 5% KCN bath, the anode being .agitated in a continuous circular path at a rate of 14 feet per minute. A voltage of '2.7 was applied for 5 seconds and raised rapidly to 4.6 for 5 seconds, then lowered back to 2.7 for 5 seconds, etc., and this cycle repeated for a total of 2 minutes (see Fig. 17). The anode appeared grey when at 2.7 volts and brown when at 4.6 volts. The anode developed a polish which was better when withdrawn after the 4:6 volt part of the cycle. When the cycles were lengthened to v15 seconds, the same results were obtained eX- cept that the polish was superior (see Fig. 18).

Results indicate that preferred polishing effect is obtained if a substantial part or all of the cycle ofI voltage and/or current density includes operation at a voltage above the polarization point and below the point at which permanent discolorations would eventually be formed. As pointed out above a short period of time is required, after applying a voltage suicient to cause eventual permanent film formation, before a permanent film is actually obtained. It appears that during this short interval, enhanced polishing is obtained because of increased dissolving action on the highest protuberances. In any case, during the remaining part of the cycle, which includes operation at voltages outside this region, the voltage preferably should not be maintained thus for a. period of time sufficient for formation of the permanent discoloration, etching, or spontaneous flashing. As in my previous application, Serial No. 474,437, the region of permanent discoloration is that voltage region above the polishing region which produces a discoloration on the metal surface, which persists when the metal is withdrawn from the bath, as contrasted with the discoloration, either brownor sometimes grey in the case of silver, whichv disappears almost instantly when the work is withdrawn from the bath or the current is cut olf or varied, to leave a polished or slightly milky surface.

Fig. 4 is for commercial bronze in a 5% KCN bath. A voltage of 1.8 was applied for l0 seconds, quickly raised to 2.4 for 10 seconds, quickly lowered to 1.8 for 10 seconds, etc. for a total of 2 minutes (see Fig. 19). This corresponds to jumps in regions A and B of the graph. A polishing effect was obtained." In another run, a voltage of 2.4'volts was applied for 5 -seconds and cut off completely for 5 seconds, reapplied for 5 seconds, etc. and a luster was observed (see Fig. 20).

Fig. 5 shows relationships for copper'in 5% KCN. A voltage of 1.8 was applied for 10 sec--V onds, quickly raised to 2.8 for 10 seconds, lowered to 1.8 for 10 seconds, etc., for a total of 2 minutes, and the polishing effect obtained (see Fig.

21). These voltages correspond to the upper part of region A and B, respectively. In another run, a voltage of 2.8 volts was applied for 5 seconds and then cut off completely for 5 seconds, reapplied for 5 seconds, etc. for a total of 2 minutes and a polishing effect was obtained (see Fig. 22)

Fig. 6 forcadmium shows relationships in 5% KCN. A voltage of 2.4 was applied for 5 seconds, quickly raised to 2.7 for 5 seconds, quickly lowered to 2.4 for 5 seconds, etc. for 2 minutes, and a polishing effect obtained (see Fig. 23). In another run a voltage of 2.7 was applied for 8 seconds and cut off completely for 8 seconds, reapplied for 8 seconds, etc. for a total of 2 minutes and a polishing effect obtained (see Fig. 24).

Fig. 7 shows relationships with cold rolled zinc `ina 5% KCN bath. A voltage of 4.5 volts was applied for 2 seconds and cut off for 2 seconds, reapplied for 2 seconds for a total of 2 minutes and a luster was obtained (see Fig. 25) In another run, a voltage of 3.6 was applied for 10 seconds, raised rapidly to 4.4 volts for 10 seconds,

lowered quickly to 3.6 volts for 10 seconds, etc.

for a total of 3 minutes and a luster was obtained (see Fig. 26). In these examples, zinc did not polish as well as, for example, silver, but a beneficial brightening was obtained that would considerably reduce the amount of bufng required.

Fig. 8 shows relationships with nickel silver in a 5% KCN bath. A potential of 4 volts corresponding to region B was applied for 8 seconds, cut olf for 8 seconds, reapplied for 8 seconds, etc. for a total of 3 minutes and a polishing effect was observed (see Fig. 27). In another run, a voltage of 1.0 volt was applied for 10 seconds, raised rapidly to 4.5 volts for 10 seconds, lowered quickly to 1.0 volt for 10 seconds, etc., for a total of 2 minutes and a luster was obtained (see Fig, 28).

It may therefore be pointed out from the figures and foregoing description of results that by positively fluctuating the potential across an electrolytic cell in which the anode surfaces are of the metal to be polished and the electrolyte contains a soluble cyanide (which is also a solvent of the metal) either continuously or abruptly, and either 1. Method for the anodio polishing of silver,

copper, cadmium, alloys consisting Vof these metals, brass, bronze, zinc and nickel silver, in an electrolytic cell containing a cyanide bath, which comprises the steps of applying a continuously unidirectional substantially constant initial electric current and potential difference between such metal, as the anode surface, and the cathode ofY the cell, positively fluctuating such potential difference, the upper limit of which fluctuation is below that potential difference at which permanent discoloration'of the anode surface would take place within the time period of the fluctuation during which it is maintained and the lower limit of which fluctuation is above that potential difference at which etching of the anode surface The period of such` would take place within the time period of the fluctuation during which it is maintained, the complete time cycle of each variation being approximately 1 to 30 seconds and continuing such positive fluctuation of the unidirectional potential difference ,between the surface which is to be polished and the cathode of the cell until at least a portion of the anode surface of the metal acquires a polish. y

2. Method for the anodic polishing of silver, copper, cadmium, alloys consisting of these metals, brass, bronze, zinc and nickel silver, in an electrolytic cell containing a cyanide bath, which comprises the steps of applying a continuously unidirectional substantially constant initial electric current and potential difference between such metal, as the anode surface, and the cathode of the cell, positively fluctuating such potential difference, the upper limit of which iiuctuation is below that potential difference at which permanent discoloration of the anode surface would take place within the time period of the uctuation during which it is maintained and the lower limit of which fluctuation is above that potential difference at which etching of the anode surface would take place within the time period of the fluctuation during which Vit is maintained, the complete time cycle of each variation being ap-V proximately 1 to 30 seconds, said periods being severally of at least such duration as to effect a selective dissolving action upon the metal protuberances of said surface, and continuing such positive fluctuation of the unidirectional potential difference between` the surface which is to be polished and thecathode of the cell until at least a portion of the anode surface of the metal acquires a polish.

3. Method for the anodic polishing of silver, copper, cadmium, alloys consisting of these metals, brass, zinc andnickel silver, in an electrolytic cell containing a cyanide bath, which comprises the steps of applying a continuously unidirectional substantially constant initial potential difference between such metal as the anode surface and the cathode of the cell, varying such potential difference through at least a part of the range between the polarization point and the region of permanent discoloration and at least a part of the region outside said range, the time of operation outside said range being kept short enough to avoid the formation of a permanent etch or a permanent discoloration, the complete time cycle of each variation being approximately 1 to 30 seconds and continuing said potential difference variation until at least a portion of the anodic surfaces of such metal acquires a polish.

4. Method for the anodic polishing of silver, copper, cadmium, alloys consisting of these metals, brass, zinc and nickel silver, in an electrolytic cell containing a cyanide bath, which comprises the steps of applying a continuously unidirectional substantially constant initial potential diierence between such metal as the anode surface and the cathode of the cell, varying such potential difference through at least a part of the range between the polarization point and l0 the region of `permanent discoloration, the complete time cycle of each variation being approximately 1 to 30 seconds and continuing said potential difference variation until at least a portion of the anodic surfaces of such metal acquires a polish.

5. Method for the anodic polishing of silver, copper, cadmium, alloys consisting of these metals, brass, bronze, zinc, and nickel silver, in an electrolytic cell containing a cyanide bath, which comprises the steps of applying a continuously unidirectional, substantially constant initial electric current and potential difference between such metal, as the anode surface, and the cathode of the cell and of imposing a continuous fluctuation of such potential difference between such metal, as the anode surface, and the cathode of the cell, the limits of which fluctuation are below that potential which will form permanent discoloration within the portion of the period of the fluctuations during which it is maintained and above that potential which will form etching` within the portion of the period of the fluctuations, during which it is maintained, the complete time cycle of each variation being approximately 1 to 30 seconds until at least a portion of the exposed anodic surface of such metal acquires a polish thereon.

6. Method for the anodic polishing of silver, copper, cadmium, alloys consisting of these metals, brass, bronze, zinc, and nickel silver, in an electrolytic cell containing a cyanide bath, which comprises the steps of applying a continuously unidirectional, substantially constant initial electric current and potential difference between such metal, as the anode surface, and the cathode of the cell and of imposing an abruptly changed fluctuation of such potential difference` between such metal, as the anode surface, and the cathode of the cell, the limits of which fluctuation are below that potential which will form permanent discoloration within the portion. of the period of the fluctuations during which itis maintained and above that potential which will form etching within the portion of the period of the fluctuations, during which it is maintained, the complete time cycle of each variation being approximately 1 to 30 seconds until at least a portion of the exposed anodic surface of such metal acquires a polish thereon.

SAMUEL E. EATON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,416,294 Eaton Feb. 25, 1947 2,473,923 Turner June 21, 1949 OTHER REFERENCES Transactions of the Electrochemical Society,

o vol. 81 (1942), pages 199-211.

The Iron Age, vol. 157, June 6, 1946, page 65. 

1. METHOD FOR THE ANODIC POLISHING OF SILVER, COPPER, CADMIUM, ALLOYS CONSISTING OF THESE METALS, BRASS, BRONZE, ZINC AND NICKEL SILVER, IN AN ELECTROLYTIC CELL CONTAINING A CYANIDE BATH, WHICH COMPRISES THE STEPS OF APPLYING A CONTINUOUSLY UNIDIRECTIONAL SUBSTANTIALLY CONSTANT INITIAL ELECTRIC CURRENT AND POTENTIAL DIFFERENCE BETWEEN SUCH METAL, AS THE ANODE SURFACE, AND THE CATHODE OF THE CELL, POSITIVELY FLUCTUATING SUCH POTENTIAL DIFFERENCE, THE UPPER LIMIT OF WHICH FLUCTUATION IS BELOW THAT POTENTIAL DIFFERENCE AT WHICH PERMANENT DISCOLORATION OF THE ANODE SURFACE WOULD TAKE PLACE WITHIN THE TIME PERIOD OF THE FLUCTUATION DURING WHICH IT IS MAINTAINED AND LOWER LIMIT OF WHICH FLUCTUATION IS ABOVE THAT POTENTIAL DIFFERENCE AT WHICH ETCHING OF THE ANODE SURFACE WOULD TAKE PLACE WITHIN THE TIME PERIOD OF THE FLUCTUATION DURING WHICH IT IS MAINTAINED, THE COMPLETE TIME CYCLE OF EACH VARIATION BEING APPROXIMATELY 1 TO 30 SECONDS AND CONTINUING SUCH POSITIVE FLUCTUATION OF THE UNIDIRECTIONAL POTENTIAL DIFFERNENCE BETWEEN THE SURFACE WHICH IS TO BE POLISHED AND THE CATHODE OF THE CELL UNTIL AT LEAST A PORTION OF THE ANODE SURFACE OF THE METAL ACQUIRES A POLISH. 